In focal plane arrays, current microbolometer uncooled technology is based on micro-machined structures that support a thermally sensitive material which involves a change in resistance in material based on incoming infrared radiation. When an array of such detectors is utilized, each of the individual detector elements is thermally isolated from its base or substrate through the utilization of relatively thin conductors, two per detector, which go from contact pads on the detector and snake about to contact pads on the substrate. The purpose of this type of mounting system is to provide a low thermal conductivity mounting device. The reason for the low thermal conductivity is to increase sensitivity. This means that all of the incident energy is converted to a signal and is not leaked out by virtue of heat exiting through the mounting apparatus.
During the detector readout procedure a current probes the resistive element and the change in resistance over an initialized value is read out. However, passing electricity or current pulse through the detector to be able to readout the change in resistance heats the detector.
Note that the voltage corresponding to the incident IR radiation is read out during the current pulse. After the readout, the current is removed and the temperature increase dissipates back to an equilibrium temperature. As will be appreciated, one must let the detector cool down between cycles which means to promote rapid cooling one needs a good thermal conductor for the mounting device for the detector. By rapidly cooling one could achieve a high frame rate.
However, using a high thermal conductivity mounting device severely reduces sensitivity. There is therefore a need for some type of mounting device that will not affect good sensitivity, yet permit high frame rate readouts.
The high frame rate is a requirement for heat seeking missiles in which there are high dynamics in the scene. In order to be able to track rapidly moving images one needs a frame rate in excess of 200 frames per second. However, because of the need for low thermal coefficient mounting devices for sensitivity, the cool down of the detector after a readout pulse is much too slow for such an application.
Moreover, other circuit elements in the IR detection system throw off heat and it is for this reason that it is desirable to thermally isolate the detector from its substrate as much as possible. This as well as sensitivity is the reason for the utilization of the thin electrical conductors which do not result in a significant amount of heat being transmitted from the substrate to the detector or vice versa.
There is however a further problem with the thin serpentine electrical conductors. The use of these thin electrical conductors in a looped or serpentine structure in which the detector in essence sits unanchored to the substrate but for the conductors takes a considerable amount of real estate. This in turn deleteriously affects the fill factor of an array of such a detectors such that the area for the detectors may be as low as 60% of the overall area. Detectors spaced apart in this position lower the overall resolution of the array, it is therefore important to be able to have a detector mounting system that thermally isolates the detector from the substrate while at the same time occupying virtually no additional space in the lateral direction.
One U.S. Patent which attempts to provide for a small pixel high fill factor uncooled focal point array is that which is described in U.S. Pat. No. 6,144,030. In this patent a thermal isolation structure is utilized to mount the detectors to a substrate. The thermal isolation structure is coupled to and spaced from the sensor which involves connection to the optically absorptive material structure. In this patent it is said that the thermal isolation structure facilitates very high fill factors even when the pixel size is shrunk below the base line 50-micron size.
In essence what this patent shows is a serpentine structure underneath the photodetector which provides the same type of thermal isolation as the serpentine conductors discussed above which are to either side of the detector. However, the thermal isolation structure while addressing fill factor fails to address the requirements of high frame rates or sensitivity.
It will be appreciated that the ideal thermal detector would have a 100% conversion of thermal energy to an observable resistance or capacitance change or the generation of a voltage potential. In order to do so it is important that the detector be thermally isolated so that all the received thermal energy is converted. It is also desirable to have the detector quickly return to an equilibrium temperature after the termination of the readout pulse. These two requirements are in opposition.
Note that for microbolometers the conversion efficiency is only in the 2 to 4% range, making thermal isolation extremely important.
To achieve increased sensitivity, low thermal conductivity supports are used to hold the detecting material. How low this can go is a problem if one wants high frame rates. In operation, after a period of integration the resistance change is determined by applying a bias voltage and measuring the resulting current or by applying a pulse of current and detecting voltage. This adds heat to the detector that must be dissipated within a short period of time for a high frame rate. For TV video frame rates at 60 Hertz, (e.g., 30 frames per second) this means that the device must settle in 16 milliseconds, e.g., the device must have a time constant of about 5 milliseconds. This requirement places a lower limit on the thermal conductivity of the support structure, and thus negatively impacts sensitivity.